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An unstable acid, H2S2O4, that is found chiefly in the form of its salts and is used as a bleaching and reducing agent. Also called hydrosulfurous acid.
(Elements & Compounds) an unstable dibasic acid known only in solution and in the form of dithionite salts. It is a powerful reducing agent. Formula: H2S2O4. Also called: hyposulphurous acid or hydrosulphurous acid
[from di-1 + thion-, from Greek theion sulphur + -ous]